1. Field of the Invention
The present invention relates to trans-dihalogenostilbene compounds useful for liquid crystal electro-optical devices, liquid-crystal compositions containing them and liquid crystal electro-optical devices using them.
2. Discussion of Background
Liquid crystal display devices have been used for watches and electronic calculators and in recent years for various applications including measuring devices, automobile meters, copying machines and cameras. Accordingly, various functions including a wide temperature range for operation, a low voltage for driving, a quick response and chemical stability, are required for such liquid display devices.
However, at present, there is no single material which by itself satisfies all of such requirements, and it is a common practice to satisfy such requirements by a liquid crystal composition prepared by mixing a plurality of liquid crystals and non-liquid crystal materials. Accordingly, it is desired to develop a liquid crystal material or a non-liquid crystal material which is excellent in one or more functions, if not in all the required functions.
For a twisted nematic (TN) type liquid crystal cell in the field of display devices using liquid crystal, in order to prevent formation of an interference fringe on the cell surface which causes deterioration of the outer appearance of the liquid crystal cell, it is necessary to set a certain specific value for the product of the anisotropy of refractive index (.DELTA.n) of the liquid crystal material filled in the cell and the thickness (d) of the cell. Since the value of .DELTA.nd is set at a certain specific level, it is possible to minimize the value d by using a material having a large value .DELTA.n. If the value of d is minimized, the response time (.tau.) can be minimized in accordance with a well known formula of .tau..alpha.d.sup.2.
Thus, a liquid crystal material having a large value of .DELTA.n is a very important material useful for the preparation of a liquid crystal cell having a high response speed and yet free from an interference fringe.
On the other hand, practically feasible liquid crystal material is in many cases prepared by mixing several or more components comprising compounds having a nematic phase around room temperature and compounds having a nematic phase in a temperature range higher than room temperature, and is usually used in the form of a liquid crystal composition.
Further, it is known that the response time is positively interrelated with the viscosity of the liquid crystal material.
Accordingly, it is an important object to provide a material which has a large value of .DELTA.n, small viscosity and excellent compatibility with other liquid crystal materials or non-liquid crystal materials and which is chemically stable.